This invention relates to a process for preparing cephalosporin compounds. In particular, it relates to an enzymatic process for converting certain peptides to 3-exomethylenecepham-4-carboxylic acids comprising the use of isopenicillin N synthetase (IPNS).
3-Exomethylenecepham-4-carboxylic acids and esters thereof represented by the following general formula are known. ##STR1##
For example, Chauvette et al., J. Org. Chem., 38, 2944 (1973) describe several 7-acylamino-3-exo-methylenecepham-4-carboxylic acids and esters as well as the 3-exomethylenecepham nucleus, 7-amino-3-exo-methylenecepham-4-carboxylic acid. U.S. Pat. No. 3,932,393, issued Jan. 13, 1976, describes a reductive process for preparing 3-exomethylenecepham compounds, in particular, the 3-exomethylenecepham nucleus.
3-Exomethylenecepham compounds are useful intermediates to 3-methyl-3-cephems, e.g., cephalexin, U.S. Pat. No. 3,507,861; to 3-halo-3-cephems, e.g., cefaclor, U.S. Pat. No. 3,925,372; and to 3-hydroxy- and 3-alkoxy-3-cephems described by U.S. Pat. No. 3,917,588.
Prior to this invention, the 3-exomethylene-cepham compounds were obtained by chemical synthesis. Owing to the importance of these cephalosporins as intermediates to clinically useful antibiotics, an alternative process for their preparation is highly desirable.
The cyclase isopenicillin N synthetase (IPNS) is well known for its ability to cyclize .delta.-(L-.alpha.-amino-adipoyl)-L-cysteinyl-D-valine to isopenicillin N via cell-free extracts of the enzyme from Acremonium crysogenum as well as other sources. Recent studies carried out with the purified enzyme have shown that the enzyme is capable of forming the cepham ring as well as the penam ring, J. E. Baldwin et al., J. Chem. Soc., Chem. Commun., 1984, 1211. Because of the importance of the penicillins and cephalosporins in therapy, considerable effort has been undertaken to explore the use of IPNS in the preparation of .beta.-lactam compounds.